Carburizing is an effective method of increasing the surface hardness of low carbon, unalloyed, or low carbon, low alloy steels. Typically, steel articles are placed in an atmosphere containing carbon in an amount greater than the base carbon content of the steel and heated to a temperature above the austenite transformation temperature of the steel. After the desired amount of carbon has been diffused into the article, hardness is induced by quenching.
Gas carburizing is efficient, controllable, and one of the most widely used methods of generating a carbonaceous atmosphere for carburizing. However, most commonly used gas mixtures typically contain small amounts of oxygen which tend to form surface oxides with one or more of the steel elements which have a strong affinity for oxygen, such as silicon, chromium and manganese. Oxides that form along grain boundaries, i.e., intergranularly, extend inwardly from the surface and have a harmful effect on the mechanical properties of the carburized article.
Conventional carburizing processes typically avoid the formation of case carbides, and generally produce an essentially carbide free martensitic structure. Even when case carbides are involuntarily formed, characteristically there is a thin surface layer void of carbides. This is due to the oxidation of carbide forming elements in the surface layer.
The detrimental effects of oxidation during gas carburizing have been known for a long period of time. Heretofore, when designing a carburized article, it has been necessary to consider the reduction in bending fatigue strength attributable to intergranular surface oxides. To avoid reduction in bending fatigue properties, it has been necessary to physically remove intergranular surface oxides formed during carburizing by machining or grinding, or prevent such surface formations by removing oxygen compounds from the carburizing media. These alternatives are costly. A 1978 article authored by Ruth Chatterjee-Fischer, "Internal Oxidation During Carburizing and Heat Treating," Metallurgical Transactions A, published by American Society for Metals and the Metallurgical Society of AIME, Vol. 9A, Nov. 1978, pp. 1553-1560, reported that, when employing conventional carburizing processes, the presence of silicon in the parent metal is a prime contributor to the formation of oxides at the surface of the article. However, while identifying that lowering silicon is a solution to the problem of surface oxidation, the paper fails to recognize any interrelationship between low silicon amounts and the enhanced formation of surface carbides by nonconventional carburizing methods.
A low silicon carburizing steel, developed by Sanyo Special Steel Co., Ltd. is described in Japanese Patent Publication No. 57-23741. The Sanyo reference teaches that low silicon, i.e., 0.06% to 0.12%, when combined with relatively high carbon and chromium amounts will accelerate carbon diffusion and thereby reduce carburizing time. This reference limits the amount of chromium in the steel composition to intentionally avoid the formation of case carbides. Furthermore, this reference fails to link the influence of a low silicon composition with the formation of surface oxides and carbides during carburizing. The Sanyo low silicon steel composition described in the above publication is no longer in commercial production.
Another low silicon carburizing steel intended for use in applications wherein the formation of case carbides is purposefully avoided, was developed by Kobe Steel, Ltd. Kobe's composition is described in Japanese Patent Publication No. 61-253346 and, as described therein, was developed for use in a gear that is heat treated after carburizing to improve the hardness of the root areas of gear teeth. To assure favorable surface hardenability after carburizing, Kobe intentionally limits the amount of carbide forming elements in the steel composition to prohibit the formation of surface carbides during carburizing. Also, the amounts of silicon, manganese and chromium in the composition are restricted to prevent the formation of granular oxides which lower the heat treatability of the carburized surface layer. This reference also does not disclose any interrelationship between low silicon amounts and the beneficial formation of surface carbides.
A low silicon steel composition intended for carburized articles in which near surface carbides are formed is disclosed by Daido Special Steel Co. in Japanese Patent Publication No. 61-104065. Daido, like the above references, recognizes that restricting the amount of silicon in the steel composition is beneficial for reducing intergranular oxides. However, this reference also teaches that more than 1.2% chromium is essential to form carbides at or near within a zone extending inwardly from the surface to a depth of 0.1 mm below the surface. This reference also fails to recognize that in addition to its effect on oxidation, silicon effects surface carbide formation and, by limiting the amount of silicon in a carburizing steel as taught by the present invention, surface carbides may be easily formed with significantly smaller amounts of chromium.
Conventional gas carburizing processes, as discussed above, generally attempt to prevent the formation of case carbides. A nonconventional carburizing process for intentionally forming carbides in the case is described in Canadian Patent 610,554, "Carburization of Ferrous Alloys," issued Dec. 13, 1960, to Orville E. Cullen. Cullen teaches a method for carburizing low alloy steel by repeatedly carburizing and rapidly cooling the steel article. However, Cullen's method is very lengthy, requiring on the order of about 42 hours to complete, and is therefore quite expensive. Further, Cullen's process does not suggest a solution to the problem of surface oxide formation.
More recently, a two stage carburizing process was described in U.S. Pat. No. 4,202,710, issued May 13, 1980, to Takeshi Naito, et al. That process forms spheroidal carbides within a region between 0.1 mm and 0.4 mm below the case surface, but fails to provide a high density of carbides on the outer surface of the carburized case. As a result, articles formed by this teaching must initially wear, or be machined, down to the carbide rich zone beginning 0.1 mm below the surface before the enhanced wear and contact fatigue properties of the carbide microstructure, such as pitting and spalling resistance, can be advantageously utilized. Also, like Cullen, this process fails to offer a solution to the problem of surface oxide formation.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a carburized steel article which is essentially free of surface intergranular oxides and which has a high ratio of carbides on the surface without requiring any removal of material from the carburized surface. It is also desirable to provide a process which will form this article in a short treatment time, and is economical and controllable.